This invention relates to a process for modifying the surface of articles made of certain polyolefins and polyolefin blends to improve adhesion of paints and similar protective or decorative coatings thereto.
Polyolefins such as polyethylene, polypropylene, and ethylene/propylene copolymers have found many applications, for example, in household articles, toys, appliances, furniture, and automobile parts. It is often desirable to apply to the exterior surface of a polyolefin article a protective or decorative coating, especially paint, but it is necessary to first prepare the surface either by mechanical treatment or by use of a primer. It is known to use as primers various ultraviolet sensitizers, for example, benzophenone or trichloroethylene, then to irradiate the surface with ultraviolet light. It is also known to use primers which do not require ultraviolet irradiation, for example, a chlorinated polyolefin sold by Eastman Kodak Company.
While most prior art methods of chemical surface modification are adequate for certain polyolefin materials, they are not entirely satisfactory for others. Thus, it is easier to prime the surface of a vulcanized EPDM copolymer than of polypropylene or polyethylene. EPDM copolymers are either copolymers of ethylene, propylene, and a nonconjugated diene having only one polymerizable double bond or copolymers of ethylene, propylene, a diene having only one polymerizable double bond, and a nonconjugated diene in which both double bonds are polymerizable.
Since it is frequently preferred to use a polymer blend, rather than a single polymer, in fabricating a polyolefin article, the selection of a suitable primer will thus depend on the composition of the blend. For example, blends of polypropylene with EPDM copolymers have been found to be particularly useful for automobile bumpers, bumper inserts, and automobile trim. Because those automobile parts are exposed to varying atmospheric conditions, to rain water, and to hot water in automatic car wash installations, good adhesion of paint is particularly difficult to achieve. If the paint finish blisters of peels, the part is unsatisfactory both to the car manufacturer and to the car owner. Accordingly, there is great need in the industry for a primer that would provide good paint adhesion, under dry and wet conditions, to surfaces or articles made from a polyolefin or polyolefin blend in which polyethylene or polypropylene is the only or the predominating polymer.